Air core gauges have achieved a considerable amount of commercial success over the past several decades basically because of their simplicity and the elimination of many of the mechanical parts associated with purely mechanical instrumentation. Air core gauges usually use a plurality of coils mounted at various angles with respect to one another that are energized in accordance with the value of a remote condition to provide a resultant magnetic field for rotating a magnet fixed to a pointer shaft. A pointer on the shaft is thus rotated to an angular position indicating the value of the sensed condition. These devices are frequently used as speedometers, tachometers, fuel gauges, pressure gauges, temperature gauges and voltmeters.
In some cases these air core gauges include two orthogonally positioned coils that are energized in such a way that the magnetic field produced by one coil increases while the other descreases in response to a changing signal from a remote condition until one coil is providing a maximum field in one direction, and the other a minimum field in the other direction. Under these circumstances the resultant magnetic field can be shifted 90 degrees permitting the same maximum theoretical pointer movement.
In another similar type of known air core gauge, two primary orthogonally related coils are provided in which the magnetic fields in both coils increase and decrease in response to the changing signal representing the sensed condition. In these instruments a third coil is positioned to provide a relatively fixed force in opposition to one of the primary field forces so that the summation of the three fields deflects the pointer magnet and pointer. The resultant magnetic field can be shifted 90 degrees and thereby provide 90 degrees of pointer movement.
Normally in such instruments the pointer returns to zero by gravity when the DC power supply is turned off. However, there are instances, such as present day military vehicular specifications, where a positive zero pointer return is required regardless of the attitude of the associated vehicle, i.e. vertical-horizontal-upside down, etc. Moreover, the same military specifications require instrument voltage regulation because supply voltage variation frequently will cause pointer movement and error.
It has been suggested in the part by Faria Corporation to provide a secondary fixed magnet adjacent the pointer rotor magnet aligned so it returns the pointer to zero when the supply is shut off. However, this gauge requires a voltage regulator to eliminate pointer movement with changes in supply voltage and also requires a transistor amplifier stage to increase the signal from a remote condition.
The Reenstra U.S. Pat. No. 4,492,920 shows the basic concept of compensating for the return biasing effect of a zero return magnet with an additional coil wound around the bobbin of a two coil air core gauge. This system requires an extra coil to effect compensation regardless of the number of driving coils.
It is a primary object of the present invention to provide an improved air core gauge with an all-attitude zero pointer return without the requirement for voltage regulation.